In a conventional full-bridge soft-switch circuit, because of the advantages of a simple circuit and control, soft switch being achieved easily by switch, high circuit efficiency, low EMI, etc., soft-switching has been widely used in the area of converters. However, because an auxiliary inductor is added therein, a diode may cause a relatively large voltage spike and oscillation during the reverse recovery of a diode at the secondary side, thereby increasing the loss of a diode switch such that the EMI of circuit increases. If boosting the withstand voltage of diode, the reverse recovery time of diode will be longer which makes circuit performances even worse.
In order to eliminate the influence caused in the recovery of a backward diode so as to enhance the circuit reliability, Chinese Patent Application No 03114296.6, titled “Soft Switch Full-bridge Phase-shift Circuit with Resonant Inductance Voltage Clamp”, (inventors: Zhang Huajian, Lv Minghai, Wang Guoyong, Huang Boning, Publication Date: Nov. 12, 2003) discloses a novel resonant inductance voltage clamping soft switch full-bridge phase-shift circuit. It uses the clamping winding of a resonant inductor to resolve the problems caused during reverse recovery of an output diode such that the circuit, while maintaining the inherent soft switch characteristics of full bridge circuit, greatly reduces oscillations caused by the reverse recovery of the output diode. In a present configuration, as shown in FIG. 1, a resonant inductance branch connects a resistor Rc in series. Rc in series ensures that the circuit, during each switch cycle, consumes extra energy of the resonant inductor and eliminates the influence caused by the recovery of a backward diode, and ensure that the clamping diodes D5 and D6 are zero current switches. In this way, it can enhance the reliability of the clamping circuit considerably. However, the current stress di/dt or voltage stress dv/dt on the added resistor Rc is relatively large, and the peak is relatively high, such that there are certain effects against the reliability of resistor Rc. For the clamping resistor Rc, the average energy needed to be consumed in whole procedure is substantially definite, and is determined by the characteristics of the entire circuit. The average loss of the resistor can meet requirements under selected conditions, but instantaneous power is different. In particular, at the instant of conducting current, the instantaneous power is very large, while when the current is zero, the resistor has no loss. Resistor instantaneous stress is an important factor for both the selection and lifetime of resistors. If the instantaneous power of a resistor is excessively large it will reduce the lifetime of the resistor, and potentially damage the resistor. Therefore, in the situation of maintaining total loss unchanged, it is desired that the instantaneous power is as lower as possible.